The present invention relates to a method and apparatus for quantifying sound emanating from a product in a production line setting as a quality assurance check; and more particularly, for measuring sound intensity in order to determine the sound power emitted by a source in a dynamic environment of ambient background noise.
In practice, sound emission testing of certain products must be accomplished to assure quality control or verify noise specification standards for these products. For example, motors intended for use in aircraft cockpits, where cabin noise levels are strictly controlled, must meet strict acoustic specifications. It is also extremely desirable to measure the sound emanating from mechanical devices in operation, such as transmissions, compressors, etc. as a post-production quality assurance test to detect excessive vibration indicative of manufacturing defects. Catching such defects before products enter service avoids expensive recall programs and adverse customer reactions.
Problems faced by artisans in the sound measurement art include being able to quickly and conveniently evaluate sound sources in a production setting where masking levels of background noise are typically present. The ambient sound field, the region into which sound emanates, is affected by the propagation of extraneous sound i.e. noise, as well as, the presence or absence of reverberation, reflection and scattering. Sound testing without unacceptable loss of accuracy usually requires that each product be taken into an anechoic test chamber or other well defined acoustic space to control all but source contributions to the sound field. However, this limits the speed at which products can be tested. Prior emphasis in the sound detection art has been to control the ambient sound field, i.e. eliminate background noise rather than compensate for the existence of a noisy environment.
Heretofore, artisans have recognized but ineffectively utilized a noise compensating technique based on the direct measurement of sound intensity using a matched two microphone probe. The technique is based on the principle of conservation of sound field energy, which provides a general theoretical basis underlying a total sound power approach to characterizing a sound source exclusive of background noise.
The sound power W, representing the rate of transmitted sound energy, is mathematically expressed as the following closed surface integral of sound intensity flux: ##EQU1## where I is the instantaneous vector sound intensity indicative of the instantaneous rate of sound energy flowing through an arbitrary closed surface, A, enclosing a sound source; and dA is a differential surface vector directed outwardly normal to surface A. In short, equation (1) states that the flux of sound energy into closed surface A is equal to the flux of sound energy out of closed surface A for all sound originating outside the closed surface A. Performing the integral, "averages out" all vector sound intensity contributions due to those sources not enclosed by surface A. The integrated sound power inherently characterizes only those sources enclosed by surface A.
It is well known that an expression for instantaneous acoustic intensity, I can be derived in the frequency domain dependent only upon the Fourier transforms of pressures as measured using a two microphone technique. The reader is referred to U.S. Pat. No. 4,532,807 entitled "Method and Apparatus for Detecting Sound Source" by Tomita et al. which is incorporated by reference herein. This patent provides the details of such a derivation. This well known expression for the magnitude of the sound intensity vector, I (f), in the frequency domain at a position midway between two microphones, A and B, is given as follows: ##EQU2## where .rho. is the density of air, .DELTA.r is the separation between the microphone pair constituting the probe, P.sub.A (f) is a scalar pressure measured at microphone A after Fourier transforming the signal to a frequency spectrum, P.sub.B *(f) is the complex conjugate of a similarly transformed measurement at microphone B and "Im" indicates that only the imaginary portion of the complex cross power spectrum given by P.sub.A (f)P.sub.B *(f) is considered in computing sound intensity. The sound intensity vector, in this form, is immediately derivable from a Fourier Transform (FT) of measurements taken at microphone A and microphone B of each probe; as all other contributions are taken to be constant for a given measurement.
The necessary prerequisite for implementing sound intensity as derived in equation (2) directly into the sound power calculation of equation (1), in a computationally straight forward manner, lies in gathering the pressure measurements from a sufficiently distributed plurality of probes enclosing the sound source over the same time interval.
Until now, conventional measurement techniques, like that disclosed in the above identified U.S. Pat. No. 4,532,807, have relied on scanning a measurement surface over various differing time intervals with a single probe rather than using a plurality of probes to collect measurements over the same time interval. This is because commercially available matched two microphone probes are quite expensive; ranging in cost from $5000 to $10,000 each. Such a precision microphone pair is generally carefully matched by physically modifying the probe itself, resulting in an expensive probe designed for a special purpose. It would be labor intensive and far too costly to attempt to delicately adjust a plurality of such costly probes to have identical gain and phase characteristics in this manner. Based on these limitations, single probe scanning is typically used to obtain a distributed plurality of measurements necessarily taken over varying time intervals which must then be averaged to estimate a spatial sound intensity distribution. Such distributions of pressure or sound intensity are depicted using contour maps to graphically identify localized sound sources as disclosed in the subject patent. This process is too slow for on-line product sound testing.
It is recognized that speed as well as effective background noise elimination can be obtained for on-line product sound testing by determining the total sound power emitted by a product directly according to equation (1). However, heretofore collecting measurements over the same time interval using a plurality of correlated probes has been unduly prohibitive. Providing such as useful plurality of measurements necessitates calibrating each microphone pair comprising each probe in the plurality in a simultaneous and practical way.
The present invention has for an object the elimination of the drawbacks mentioned above and provides a method and apparatus for rapidly determining the total sound power emitted by a source in an environment of background noise. The apparatus provides an effective noise elimination feature using a convenient, portable, multiprobe array equipped with inexpensive, rugged, replaceable, microphone pairs which can be easily calibrated for rapidly collecting a plurality of measurements over the same time interval in a substantially lossless acoustic environment.